The present invention relates to a fluid friction clutch, and more especially to a fluid friction clutch for driving a fan in a liquid-cooled internal combustion engine wherein a viscous fluid is circulated between a reservoir and a working chamber to transmit torque between a drive disk and the clutch housing.
Clutches of this type are known, for example, from German Pat. Nos. 12 84 186 and 12 86 350 and are used preferably as cooling fan transmissions to control the temperature of the circulating cooling media in an automotive internal combustion engine. When the clutch is disengaged, all of the viscous fluid is in the reservoir, and the housing of the clutch with the fan attached thereto is entrained only as the result of bearing friction between the clutch housing and the drive shaft and friction produced by air located in the working gaps between the clutch housing and the drive disk. In the process, especially when ball bearings are used a relatively low rpm of the clutch is established, which in the case of high capacity truck cooling fans may drop to very low values, for example, 300 rpm, because of the high counter momentum. With rising temperatures of the cooling medium, the valve to the working chamber is opened by means of a suitable, thermostatically actuated servo mechanism, and the viscous fluid is forced into the working chamber as a result of the pressure prevailing in the reservoir. This pressure is a function of the output velocity of the clutch. Simultaneously, the fluid entering the working chamber is moved back into the reservoir via a baffle, so that a circulation of the fluid is obtained. The volume of the viscous fluid pumped from the working chamber is a function of the relative rpm or relative circumferential velocity between the drive disk and the clutch housing, i.e., the driving and driven rpm of the clutch.
It follows from these relationships that, in the case of a low output velocity of the clutch, the flow of the fluid into the working chamber is poor or delayed and that, while at the same time, with a correspondingly high relative velocity between the driving and driven sides of the clutch, there is a relatively rapid movement of the fluid from the working chamber. Both of these phenomena are disadvantageous, especially in the case of high viscosities of the viscous fluid, for example, in excess of 0.015 gm/sec, or 15,000 centistokes, because they may result in a delayed actuation of the clutch (starting of the fan) and thus in the overheating of the internal combustion engine. A lag in this actuation is particularly critical during the cold starting of the internal combustion engine, in view of the rapid rise in the engine output. The cold fluid is highly viscous and therefore passes through the valve orifice only with difficulty. With a fan having a large diameter and a high driving torque, lags in actuation of several minutes may occur, so that the cooling of the internal combustion engine during the rapid rise in load is no longer assured. For low output velocities of the clutch, i.e., low rpm of the fan, this results, as mentioned hereinabove, in decisive disadvantages concerning the actuation behavior of a fluid friction clutch of this type. On the other side, low output velocities are wanted, because they favor the warm up of the engine, save fuel and reduce the fan noise.